The invention relates to an optical amplifier, a hybrid assembly and a method of making the hybrid assembly. In particular, the invention relates to a doped glass amplifier and to a hybrid assembly including a doped glass amplifier and an optical pump, together with a method of manufacture of the same.
FIG. 1 is a schematic diagram of a known erbium-doped fiber amplifier. In such an amplifier, a length of erbium-doped optical fiber 10 is provided to carry out the amplification. Signal light transmitted along a signal optical fiber 12 and pump light from a pump light source 14 are combined in an optical coupler 16 and sent along the erbium-doped optical fiber 10. In the fiber, the pump light excites atomic states of the erbium atom to create an inversion i.e. a situation in which higher energy states have a greater occupancy than lower states. In this situation, the signal light can create stimulated emission of light in phase with itself so that the erbium-doped fiber acts as an optical amplifier of the signal light.
Such erbium-doped fiber amplifiers are widely used, especially in transmission backbone systems in which optical signals have to be transmitted down great lengths of optical fiber. In such transmission backbone systems, the high cost of erbium-doped fiber amplifiers is not an issue.
However, there is an increasing need for amplifiers in smaller, local systems. These may range from metro systems inter-connecting a small area, to switched backplane systems inter-connecting a business or access systems delivering high bandwidth optical fiber connectivity to end users. In such systems, the high cost of conventional erbium-doped optical fiber amplifiers is a real issue and prevents wide spread use of such amplifiers.
Moreover, another difficulty with erbium-doped optical fiber amplifiers is that the signal is input along a single optical fiber, the input optical fiber 12 of FIG. 1. Although multiplexing techniques can be used to direct a plurality of signal down a single optical fiber for amplification, such techniques can be inconvenient. Often a separate erbium-doped fiber amplifier is required for each optical signal.
U.S. Pat. No. 5,982,973 to Yan et al describes a planar optical waveguide device, in which a specific glass composition is used. In an experimental result a net optical gain of 4.1 dB was obtained in a planar waveguide device having a length of 10 mm. However, such a device would simply replace a conventional erbium-doped optical fiber.
It is possible to integrate a number of optical components on a substrate to create a so-called optical hybrid. For example, U.S. Pat. No. 5,534,442 to Parker et al describes a process for use in manufacturing opto-electronic components in a hybrid module form. A hybrid substrate of silicon is provided with v-grooves for locating input and output optical fibres and a number of optical and electronic components are mounted on the substrate and interconnected. A number of refinements to this technique are known. For example, U.S. Pat. No. 5,574,811 to Bricheno et al, describes a method of aligning an optical fiber with a laser mounted on a silicon motherboard by using a silicon platform to which the end of the fiber is secured.
However, it is not easy to integrate Er-doped fiber amplifiers with such modules, since the fibers cannot simply be mounted directly to the substrate.
One approach has been suggested by Regener et al in U.S. Pat. No. 5,726,796. This patent describes an optical amplifier in which a waveguide is integrated on a substrate. The waveguides have a spiral configuration, and are integrated with optical couplers and a pump light source mounted on the substrate.
However, such approaches do little to reduce component count or simplify manufacturing of hybrid optical modules. Accordingly, there remains a need for an improved optical amplifier, an improved optical hybrid assembly and a corresponding method of manufacture.
According to the invention, there is provided an optical amplifier comprising a slab having opposed top and bottom surfaces and at least one edge surface extending between top and bottom surfaces, the slab defining an optical input for receiving light to be amplified on an edge surface, at least one optical waveguide extending from the optical input thorough a doped region of the slab for transmitting and amplifying the light received from the optical input, an optical output on an edge surface for delivering amplified light from the optical waveguide, and a pump light input for receiving pump light, wherein the slab is at least partially transparent and configured to distribute, through the slab, pump light incident on the pump light input over the length of the at least one optical waveguide.
The optical amplifier according to the invention can be readily incorporated into larger systems, and in particular into hybrid amplifier modules. A slab can be readily mounted on a substrate, which is not true of a fiber, and perhaps more importantly a slab optical amplifier with pump light input separate from and spaced away from the signal input/output is relatively straightforward to integrate with further components.
In prior art waveguide amplifiers, a coupler has been provided to couple both the signal light and the pump light into the input end of the waveguide. In contrast, in an optical amplifier according to the invention the pump light is pumped not into the end of the waveguide, but passes through the slab into the waveguide along the length of the waveguide. The omission of the optical coupler may simplify manufacture of systems using the optical amplifier according to the invention.
Moreover, there is no need to provide a pump light source that is capable of directing its light output down a narrow optical fiber. Instead, a broad stripe pump light source may be used; such pump light sources can provide more power at a given cost than the pump light sources conventionally used.
Also, separation of the pump light inputs and the signal light input and output means that each can have a reflection or anti-reflection coating appropriate to their own requirements. In particular, reflective material may be provided on the outer surface of the slab away from the optical input and output and the pump light input. The reflective material may multiply reflect pump light within the slab to distribute it across the interior of the slab and hence to provide pump light along the optical waveguide.
The pump light may be distributed over substantially all of the length of the optical waveguide since amplification can be most efficient if the whole waveguide is pumped. However, it is not essential that the pump light is absolutely evenly distributed.
The dopant concentration in the doped region may exceed 5xc3x971019 cmxe2x88x923 for providing significant amplification in a short length.
A significant advantage of the approach according to the invention is that a plurality of waveguides may be provided. The waveguides may be arranged in parallel with each other. In this way, a single optical amplifier and a single pump light source may amplifier the signals of a significant number, say ten, of input optical fibers. This approach can greatly reduce the cost of amplification because in a system in which signals on ten optical fibers need to be amplified only one amplifier is needed rather than ten.
Preferably, the slab has opposed top and bottom faces spaced apart by a distance substantially less than the smallest linear dimension of the top and bottom faces, and a pump edge face extending between the top and bottom faces at the periphery of the slab, the pump light input being on the pump edge face.
The form of a slab allows good distribution of pump light from the pump light input to the waveguide since the pump light can be retained in the slab by total internal reflection from the top and bottom faces. Light can bounce in the slab in a zigzag pattern. Furthermore, the use of the form of a slab allows good heat sinking, since heat can be removed from the top and bottom faces. The slab may be flat, which may allow easier mounting on a substrate.
A number of techniques may be used to assist in the distribution of pump light in the slab. The slab may be substantially transparent with pump light is contained between the top and bottom faces. The pump edge face having the pump light input may be oblique so that pump light incident on the pump edge face is refracted to pass through the slab in a zig-zag path. A layer of refractive material may be provided on the pump edge face to guide the pump light into the glass slab so that it takes a zig-zag path through the glass media surrounding the waveguides.
The slab may comprise an undoped substrate and a highly doped layer on the top face of the undoped substrate.
A patterned layer of high refractive index material may define the optical waveguide. The pattered layer of high refractive index material may be arranged between the undoped substrate and the highly doped layer. The patterned layer may be Silicon Nitride.
The slab may be mounted on a further substrate; this need not necessarily be transparent.
In embodiments, the slab may have the form of a parallelipiped having planar top and bottom faces, opposed end faces normal to the top and bottom faces, a pump edge face being inclined with respect to a normal to the top and bottom faces, and a further edge face opposed to the pump edge face.
The at least one optical waveguide may extend between opposed end faces of the slab. There may be a plurality of waveguides.
In embodiments, the waveguides may extend in parallel between the opposed end faces. In this way, an optical amplifier in accordance with the invention can provide a plurality of optical amplifiers in a single body.
The at least one optical waveguide may take a convoluted path between opposed end faces of the slab for increasing the optical path length of the optical waveguide compared with a direct path. This increase in path length may be required, in particular for optical amplifiers operating in the L-band.
A further benefit of this approach for L-band amplification is that suitable materials, for example glasses, can be selected to greatly reduce up conversion in the L-band amplifier. Up conversion is a process in which electron in an excited state of an atom can be brought still higher energy level without emitting stimulated emission. Accordingly, the occupancy of the energy level that would provide the stimulated emission is lower, reducing the amplification efficiency. By choosing appropriate materials freely, it is possible to reduce these effects. Materials can be more freely chosen since it is not required to draw an optical fiber from the glass material.
In embodiments, the undoped substrate is of glass, although a number of other alternatives are also suitable including for example ceramics, glass-ceramics or a semiconductor substrate such as silicon.
The highly doped layer is likewise preferably of glass. Again, a number of alternative materials are also suitable. For example, a sol-gel process may be used to prepare a silica on silicon waveguide.
There may be a high dopant concentration of rare earth dopant, for example 1xc3x971020 cmxe2x88x923, in the highly doped glass layer. Such high dopant concentrations are very difficult to achieve with fiber amplifiers because the physical properties of the glass with high dopant concentrations are not suitable for drawing optical fibers. Accordingly, it is much easier to manufacture a high dopant concentration glass slab than high dopant concentration optical fibers. The dopant may alternatively or additionally be Thulium, Yttrium or some other species depending on the wavelength of the signal to be amplified.
A layer of high refractive index material may be provided to define the optical waveguide. The material may be silicon nitride, which may be arranged between the undoped substrate and the high doped glass layer.
The slab may be doped only in the optical waveguide, or only in a layer containing the optical waveguide. Alternatively the whole slab may be doped.
The slab may have reflective material for reflecting back pump light arranged on the outer surface of the slab away from the optical input and output and the pump light input.
The waveguide may extend across the slab from the optical input to the optical output. Alternatively, the optical input may also function as the optical output; light input at the optical input may pass down the waveguide, be reflected at the other end of the waveguide and pass back down the optical waveguide to emerge from the slab at the same location as the optical input.
A further improvement is obtained by providing a half wave plate along the path of the optical waveguide. This may be done, for example, by providing a groove in a doped glass slab and inserting a half-wave plate therein, alternatively, the half-wave plate may be integrally formed with the doped glass slab. An advantage of the half wave plate is that it exchanges the xe2x80x9cTExe2x80x9d and xe2x80x9cTMxe2x80x9d mode fields. Accordingly, it can compensate for any effects caused by polarization dependent propagation in the waveguide. Preferably, the half-wave plate is half way down the optical waveguide to more accurately cancel any polarization effects.
In another aspect there is provided an optical amplifier comprising: an at least partially transparent body defining an optical input for receiving light to be amplified, at least one optical waveguide extending from the optical input thorough a doped region of the body for transmitting and amplifying the light received from the optical input, an optical output for delivering amplified light from the optical waveguide, and a pump light input for receiving pump light on the outer surface of the body laterally of the waveguide; and reflective material, on the outer surface of the body away from the optical input and output and the pump light input, for multiply reflecting pump light within the body to distribute it across the body so that pump light incident on the pump light input is distributed over the length of the at least one optical waveguide.
The reflective material on the outside of the body causes the pump light to be multiply reflected within the body to distribute pump light within the body and thus over the length of the optical waveguide or waveguides.
In another aspect an optical hybrid assembly has a hybrid substrate, an optical amplifier as described above mounted on the substrate, and an optical pump mounted on the hybrid substrate adjacent to the optical amplifier arranged to direct pump light into the pump light input.
The term xe2x80x9chybrid substratexe2x80x9d is intended to mean a substrate of a hybrid assembly, and not to imply that the substrate itself need be hybrid. The hybrid substrate may be of silicon. Alternatively, any suitable hybrid substrate material may be used.
The optical amplifier described above is particularly suited to mounting in such a hybrid assembly. The amplifier can be handled by conventional chip mounting techniques, for ease of manufacture. In this technique the active components are mounted face down on the hybrid substrate. The pump light source may be a broad band semiconductor emitter emitting light, not at a single point but over an extended edge. Such chips are considerably cheaper than those required to direct light into an optical fiber, and accordingly much higher pump powers may be provided for the same cost than in previous arrangements.
The hybrid assembly may further comprise a carrier substrate having at least one mating member and carrying at least one optical fiber, wherein the hybrid substrate carries at least one mating member for interlocking with the at least one mating member of the carrier substrate to locate the carrier substrate on the hybrid substrate and to align the at least one optical fiber with the at least one waveguide.
The hybrid assembly preferably comprises at least one input optical fiber located to direct light into the input of the at least one waveguide and at least one output optical fiber located to receive light emitted from the output of the at least one waveguide.
The pump light source may be a broad stripe semiconductor emitter. Alternatively any suitable pump light source may be used.
In a further aspect the invention may provide an optical communications system comprising an optical amplifier module including a hybrid assembly as described above.
In a further aspect there may be provided a method of assembling an optical hybrid assembly comprising the steps of providing a hybrid substrate, providing a slab amplifier having top, bottom, opposed end and opposed edge faces, comprising at least one waveguide extending between opposed end faces arranged adjacent to the top face of the slab, flip-mounting the slab amplifier on the hybrid substrate with the top face of the slab amplifier against the hybrid substrate, and mounting an optical pump adjacent to the slab amplifier to pump light into the edge face of the slab amplifier.
Preferably, the method further comprises the step of mounting at least one optical fiber on the substrate in registration with the at least one waveguide. In embodiments, this step may be carried out by mounting the at least one optical fiber on a carrier substrate, and mounting the carrier substrate on the hybrid substrate to locate the at least one optical fiber in registration with the at least one waveguide.
The carrier substrate may be adapted to interlock with the hybrid substrate in registration with the hybrid substrate.
Preferably, the method comprises the step of adjusting the exact position of the carrier substrate after mounting it on the hybrid substrate to improve the registration of the at least one optical fiber against the at least one waveguide, and then fixing the carrier substrate to the hybrid substrate by curing rapid cure adhesive. This may be carried out by adjusting the position of the carrier substrate whilst measuring light throughput from the at least one optical fiber into or from the at least one waveguide, to find a good position for high optical throughput, adjusting the position of the carrier substrate by a predetermined amount for compensating the shrinkage of the rapid cure adhesive, and curing the rapid cure adhesive to fix the position of the carrier substrate.
The assembly may use xe2x80x9cSilicon microbenchxe2x80x9d techniques.
In a yet further aspect, the invention may provide a method of amplifying an optical signal, comprising: inputting signal light into an optical input on the periphery of a slab; passing the signal light through optical waveguide extending across a slab from the optical input; inputting pump light into a pump light input on the periphery of the slab so that pump light is multiply reflected within the slab to distribute pump light along the length of the optical waveguide; amplifying the signal light as it passes along the waveguide; and outputting amplified signal light from the doped optical waveguide.